The pilot of an experimental airplane was killed Wednesday when the wing broke off his single-engine aircraft just south of Winters, sending him spiraling to the ground, officials said.

The crash occurred around 4:15 p.m. as the plane slammed into a steep bank bordering the east side of Interstate 505 about 150 yards north of Wolfskill Road.

The pilot, who officials are not identifying pending family notification, died at the scene, said California Highway Patrol public information officer Chris Parker. He was the plane's only occupant.

Witnesses standing at the edge of an orchard near the crash site recalled watching the plane fall from the sky.

"I heard the engine making a weird sound and I looked up and saw the plane spiraling down from the sky," said Winters resident Gorden Kemp, who was out walking his dog when the crash occurred.

"I didn't hear it hit," Kemp said, saying he watched as pieces of the aircraft fell from the sky before he lost sight of the plane, which was registered to Sharman Enterprises Inc., based at Vacaville's Nut Tree Airport.

According to the FAA's N-Number registry, the 1959 Avions Fairey Tipsy Nipper T-66 was an experimental fixed-wing aircraft.

The sight of small planes flying over the area isn't unusual, Kemp said, observing that a number of aerobatic pilots practice maneuvers in the area ranging from barrel rolls to stalling and restarting their engines.

"That's what I thought he was doing," Kemp said.

"You don't expect to see something like that near your house," he added. "I didn't know where he landed until I saw the cop cars on the highway."

Firefighters, CHP officers and members of the Solano County Sheriff's Office responded to the scene, where they cordoned off the area around the wreckage of the aircraft.

The plane's wing was discovered in the median of the freeway about half a mile north of where the plane came to rest, Parker said.

Early reports indicate the pilot flew south over Winters and was en route to Nut Tree Airport when the crash occurred, Parker said. The airport does not require pilots to file a flight plan ahead of time, Parker said, so it was unknown where the victim was flying from.

Pilots who knew the longtime aviator recalled him as a good friend who had been flying most of his life, and said they were sick at the thought of such a tragedy befalling the veteran pilot.

According to Ian Gregor, public affairs manager for the FAA Pacific Region, the crash took place under "unknown circumstances."

While details remain scarce, Gregor said the NTSB will be taking the lead in the investigation and noted investigators generally post a basic preliminary report on the agency's website, www.ntsb.gov, within a week or two of an accident.

The investigation, however, typically takes months to determine a probable cause for accidents, he said.

As darkness began to fall, CHP officers were posted at the scene to provide security until investigators could arrive this morning, Parker said.